Atypical ileal hyperplasia is a spontaneous disease of hamsters characterized by massive, segmental proliferation of intestinal mucosa with invasion of underlying supportive tissues and focal granulomatous inflammation. The etiology and pathogenesis of the disease are unknown and have been difficult to study for lack of suitable procedures to induce lesions experimentally. We have now established methods for transmitting AIH in susceptible weanling hamsters and have evidence suggesting enteric bacteria are involved in transmission. The proposed research is designed to expand our preliminary work on the etiology and pathogenesis of AIH and to evaluate its suitability as an animal model for atypical mucosal proliferation in the intestine. The research is divided into 3 conceptual areas. The first concentrates on efforts to isolate and characterize the etiologic agent using a combination of in vitro culture technics and animal inoculation. The second will determine the pathogenesis of AIH by correlating serial morphologic changes in the intestinal tract at the histologic and ultrastructural level with alterations in the kinetics of mucosal epithelium monitored by radiolabeling. The third will examine the role of host immunoreactivity (cell-mediated and humoral responses) in the development of ileal lesions and the effect of immunosuppression on the biological behavior of the disease.